The Klingons were a humanoid warrior species that originated from the planetQo'noS (pronounced Kronos), an M-classplanet. One of the major powers of the galaxy, the Klingons were a proud, tradition-bound people who valued honor and combat. The aggressive Klingon culture had made them an interstellar military power to be respected and feared.

According to non-canonKlingonese, Klingons are called "tlhIngan" in that language.

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The Klingon Empire was founded some time in the 9th century by Kahless the Unforgettable, who performed many heroic feats including the unification of the Klingon people when he killed the tyrant Molor. Kahless came to be revered in Klingon society to the point of near-deification, and many aspects of Klingon culture came to revolve around an emulation of Kahless' life. (TNG: "Rightful Heir")

The warrior ethos had been an important aspect of Klingon society since the time of Kahless, but the warrior aspects became much more dominant beginning in the early 22nd century. Previously, Klingon society was regarded as socially balanced, but over time, the warrior caste gained greater prominence, to the point where the Klingons widely came to be regarded as a "warrior race." (ENT: "Broken Bow", "Judgment")

Because of their aggressive outlook, the Klingons generally had poor relations with other races after they began to move out into space. Because the worlds of the Klingon Empire were resource-poor, the Klingons developed an intense belief in the need for expansion and conquest in order to survive. The Klingons' relationship with Humans and the Federation was rocky at best. Following the disastrous first contact between Klingons and Humans in the Broken Bow incident, tense rivalries and unavoidable conflicts often developed between the two races. (ENT: "Broken Bow"; TNG: "First Contact")

A Klingon's cranial ridges dissolving

In the year 2154, the Klingons gained access to the genetic material of Human Augments and tried to adapt this genetic engineering to improve themselves. The test subjects did gain increased strength and intelligence, but then, their neural pathways started to degrade and they died in agony. One of the subjects suffered from the Levodian flu, which was modified by the Augment DNA to become a fatal, airborne, mutagenic plague that spread rampantly through the Empire, from world to world. In the first stage of this plague, Klingons lost the ridges on their foreheads and began to look more Human. With the help of a Klingon scientist named Antaak, Dr. Phlox of the Earth starship Enterprise was able to formulate a cure that halted the genetic effects of the virus in the first stage. This retained the changes in appearance, along with some minor neural re-ordering. The neural ordering caused changes in the emotional make-up of the Klingons. For example, the infected started to feel fear. Even though the infected did not develop any stage-two characteristics – such as enhanced strength, speed, or endurance – they did not die from it. This left millions of Klingons changed. These alterations were even passed on to their children. (ENT: "Affliction", "Divergence") From the 2270s onward, Klingons encountered by the Federation had their forehead ridges restored. (Star Trek: The Motion Picture)

It is not known if every Klingon in the galaxy was afflicted by the virus or when, if ever, it was cured. To date, no Klingons appearing between 2154 and the 2270s have been shown with the forehead ridges. When the Klingons Kang, Koloth, and Kor appeared on Star Trek: Deep Space Nine, however, they sported forehead ridges which they did not have during their appearances on the original series. It is unclear how they acquired their ridges in that time.

Klingons were apparently so embarrassed by the fallout from their failed attempt at genetic enhancement that they refused to discuss the incident with outsiders. Due to the secrecy of the Klingon Empire, knowledge of the change became lost over time to the general population of the Federation. By the 24th century, the reason for smooth-forehead Klingons was not widely known outside the Empire, and questions were generally met with a brusque answer along the lines of, "We don't discuss it with outsiders." (ENT: "Affliction", "Divergence"; DS9: "Trials and Tribble-ations")

Apocryphally, the game Starfleet Academy further explores the conflict between the Federation and Klingons in the years leading up to the Khitomer Accords. In the game, a series of border skirmishes are resolved when it is discovered that an unknown alien has been behind attacks on both Federation and Klingon border planets. This then helps to establish the foundation of the Khitomer talks.

By 2259 in the alternate reality, after first contact with the Empire, the Klingons had conquered and occupied two planets known to the Federation and fired on Starfleet ships half a dozen times. Tensions between the two powers were high and an all-out war was considered inevitable. During that year, before surrendering to the Federation, Khan Noonien Singh destroyed three D4-classpatrol ships on the Klingon homeworld, killing the crews of the vessels. (Star Trek Into Darkness)

Klingon society was extremely complex. Before its decline in the mid 22nd century and again in the late 23rd century, Klingon society was based on a feudal system organized around traditional Great Houses of noble lineage, to which various parts of the population owed fealty. The Great Houses are traditionally represented in the Klingon High Council, which is led by a Chancellor.

The decline of Klingon culture is demonstrated in the acts of the Klingons themselves. They stopped caring about their weapons to the point that they let them rust and even stopped caring for true honor. (ENT: "Marauders", "Judgment") Sometime after the augment virus took hold of the Klingon Empire, a new regime took control, turning the Empire into an authoritarian state that kept tabs on all who served. (TOS: "Errand of Mercy") The old ways returned in the latter 23rd and early 24th centuries respectively.

Males traditionally dominated public life in the Empire, assuming the leading roles in politics and the military with only rare exceptions. (TNG: "Redemption") A notable exception to the prohibition of women serving on the High Council came when Azetbur became Chancellor of the High Council after her father, Gorkon, was assassinated in 2293. (Star Trek VI: The Undiscovered Country) Women, in turn, traditionally dominated the household and the management of the family's affairs. (DS9: "You Are Cordially Invited") Klingon women were treated as equals, except in politics and matters of inheritance. They were prohibited by law from serving in the High Council and could not take control of their Houses unless they had the money and no male successors of the lineage. Otherwise, Klingon women were expected to exhibit the same physical prowess and lust for blood and honor as the men.

Klingon society functioned through a system of family reputation and honor. Tradition was an integral part of their lives and breaking from observances was considered a grievous insult to society, an insult not forgotten easily. An offense usually brought shame to the offender's name for several generations. The highest shame was discommendation, an action by the High Council to officially strip a Klingon of his personal or family honor. Bloodlines and relations were also taken very seriously by any "true" Klingon. Lines comprised more than mere family members. (TNG: "New Ground")

The most distinctive feature of Klingon anatomy (except in those individuals afflicted with the Augment virus) was a sagittal crest, beginning on the forehead and often continuing over the skull. The cranium was encased in an exoskeleton, which possessed a feature known as the tricipital lobe. (TNG: "Descent")

Internally, Klingon anatomy was markedly different from that of Humans. There was a great deal more multiple redundancy in their organs, a principle they called brak'lul. This allowed Klingons to survive severe injuries in battle. They had twenty-three ribs, two livers, an eight-chambered heart, three lungs, and even redundant neural function as well as multiple stomachs. Some geneticists believed that the extra organs, notably the third lung, evolved to give Klingons greater stamina on the battlefield. Klingons had relatively little knowledge of their own biology and their medicine was very poorly developed. This was largely due to their warrior traditions – a Klingon who was wounded was expected to be left to either survive through his own strength, die, or undergo the hegh'bat, a form of ritual suicide. (TNG: "Ethics"; VOY: "Lineage")

Despite the anatomical and physiological differences between Klingons and Humans, the two species had very similar nutritional requirements. Dr. Pulaski once noted that, while most Humans find Klingon food unpalatable, usually, "what kills us, kills them." (TNG: "A Matter Of Honor") However, the "tea" used in the Klingon tea ceremony seems to be an exception. (TNG: "Up The Long Ladder") Apparently, the tea concentrated some (unknown) toxic heavy elements found in the soil in which its plant of origin grew, synthesizing a poison deadly to Humans, and capable of seriously sickening Klingons, as it did.

Klingon pregnancies normally ran thirty weeks, but with mixed species, gestation times were shorter. The odds against Klingon-Human conceptions were rather high; however, when successful, Klingon and Human metabolisms sometimes clashed, causing biochemical fluctuations in the mother, which may lead to fainting. Klingon traits remained dominant for several generations, even with a single ancestor; therefore, a child even ¼ Klingon still possessed forehead ridges, if he or she carried the gene. (VOY: "Lineage")

The Klingon back and spine

Klingon feet

Klingons had ridged spines, chests and feet. (TNG: "Ethics"; DS9: "Sons of Mogh"; ENT: "Broken Bow") After birth, some Klingon infants experienced a pronounced curvature to the spine, a form of scoliosis, which was correctable by surgery. This "defect" tended to run in Klingon families, especially among females. Federation medicine, fortunately, advanced beyond that, allowing an additional choice of treatment involving genetic modification. (VOY: "Lineage")

For scenes from "Ethics" featuring actor Michael Dorn as Worf, Michael Westmore designed Klingon feet and a Klingon spine. He based the spine on the length of Dorn's actual spine, but made each single Klingon vertebra the same size as two Human vertebrae. "At the base of the spine," said Westmore, "I built a small auxiliary brain, which controlled the lower part of Worf's body [....] The producers [also] wanted me to come up with a makeup device for Worf's feet that would make the Klingon anatomy even more interesting than it already was. So I constructed a series of spines that ran down the front of Worf's feet and built a makeup appliance that looked like a horn [....] the whole effect turned out very well." (Star Trek: Aliens & Artifacts, p. 69)

Klingon children matured far more quickly than Human children. At the age of only one Earth year, a Klingon child had the appearance a Human child had at about four. By the age of eight Earth years, a Klingon attained the maturity a Human did not reach until about age sixteen. (TNG: "Reunion"; DS9: "Sons and Daughters") When Klingon children began growing into adults, they went through jak'tahla, a Klingon form of puberty. (Star Trek: Insurrection) Like other mammalian species, Klingon females were capable of lactating to breast-feed infants. (TNG: "A Matter Of Honor")

Klingons tended to live for over 150 years. Even into advanced old age, they tended to still be strong enough for combat. (DS9: "Blood Oath")

Klingons such as Kurn had the instinctive ability to sense the decision to kill by looking into the eyes of their opponents. (DS9: "Sons of Mogh") Worf did not have this ability, probably due to the fact that he was raised by Humans on Earth.

The 1977reference bookStarfleet Medical Reference Manual names the Klingon species as the Klingonoid biped or Homo Sagittarii, from the the planet Klingonii also known as Epsilon Sagittarii B. Klingons are said to be bred for efficiency in a strict, government-supervised selective breeding program.

Ritual was a very important element in Klingon society. While the Klingons were not a religious people as such, they did believe that deities existed at one time. However, Klingon warriors supposedly slew their gods, as they were considered to be more trouble than they were worth. (DS9: "Homefront")

Once a Klingon died, the spirit was considered to have exited the body, leaving behind a worthless shell to be disposed of. (VOY: "Emanations") In the Klingon death ritual, it was traditional for those on hand to howl into the sky, as a warning to the afterlife that a Klingon warrior was about to arrive. (TNG: "Heart of Glory"; DS9: "Tears of the Prophets") In some cases, a funeral dirge was sung in memory of the deceased, or friends sat with the body to protect it from predators, a practice known as ak'voh. (DS9: "The Ship")

Furthermore, a Klingon who was unable to fight, and hence unable to live as a warrior anymore, had the traditional obligation of committing the hegh'bat, which was the Klingon ritual suicide. Tradition dictated that the eldest son or a close personal friend must assist. That person's role was to hand the dying Klingon a knife so that he could plunge it into his heart, remove it, and then wipe the blood on his own sleeve. (TNG: "Ethics")

The Klingon afterlife was supposedly divided into two branches. The dishonored were taken to Gre'thor aboard the Barge of the Dead, a vessel captained by Kortar, the first Klingon. Kortar was supposedly the one who had originally killed the gods who created him and was condemned to ferry the dishonored to Gre'thor as a punishment. Once in Gre'thor, the dishonored were watched over by Fek'lhr, a vaguely Klingon-esque figure. While it may be tempting to view Fek'lhr as the Klingon equivalent of the Human devil, according to a statement made by Kang, the Klingons had no devil. (TNG: "Devil's Due"; VOY: "Barge of the Dead"; TOS: "Day of the Dove")

Those who died honorably supposedly went to Sto-vo-kor, where Kahless was said to await them. However, should a noble warrior die in a manner that might not merit a place in Sto-vo-kor, such as being assassinated in a surprise attack, he may still earn a place, if others dedicated a great battle to his name, thus showing that he had earned respect among the living. (TNG: "Heart of Glory", "Rightful Heir"; VOY: "Barge of the Dead"; DS9: "Shadows and Symbols")

Klingon rituals included the R'uustai, a bonding ceremony which joined two people together in a relationship similar to brotherhood. (TNG: "The Bonding") Klingon tradition held that "the son of a Klingon is a man the day he can first hold a blade." (TNG: "New Ground")

If a Klingon warrior struck another Klingon with the back of his hand, it was interpreted as a challenge to the death. Klingon warriors spoke proudly to each other; they did not whisper or keep their distance. Standing far away or whispering were considered insults in Klingon society. (DS9: "Apocalypse Rising")

When going into battle, Klingon warriors often sang the traditional warriors' anthem, which was essentially an invocation to Kahless and a pledge to win a good death in battle. (DS9: "Soldiers of the Empire")

When choosing a mate, it was traditional for a female Klingon to bite the male's face, allowing her to taste his blood and get his scent. (VOY: "Blood Fever")

It is likely that this tradition stems from a more primitive time in Klingon evolution. When Worf was devolving due to Barclay's Protomorphosis Syndrome, he did this to Deanna Troi and later tracked her down by her pheromones. (TNG: "Genesis") For whatever reason, this later became a deliberate tradition for women to follow.

Worf once told Wesley Crusher that, per Klingon mating rituals, "Men do not roar. Women roar. Then they hurl heavy objects. And claw at you." Of men, Worf said, "He reads love poetry. He ducks a lot." (TNG: "The Dauphin") Klingon daughters traditionally were given a piece of jewelry called a jinaq when they became old enough to select a mate. (TNG: "Birthright, Part II")

In the novel Summon the Thunder, part of the Star Trek: Vanguard series, the Klingons who had a Human appearance (descendants of the victims of the Klingon Augment virus) are referred to as "QuchHa", or "the unhappy ones". They usually served in their own units although they also were known to mix with the rest of the fleet on occasion.

In the novel Pawns and Symbols, Klingons are discovered to be color-blind in the Human sense, unable to distinguish red from black. It is also discovered that their vision extends into the ultraviolet, to 32,000 Ångströms.

In the novel Ishmael, the Klingons are described as having been economically conquered and uplifted by the Karsid empire. The Klingons then rebelled and overthrew the Karsids, obtaining their high technology. This was given as one reason for why the Klingons were the way they were, and also how they could have developed star-faring technology given their current social structure.

In Star Trek Online, most Klingons are once again enemies with the Federation by 2399, having taken advantage of the Romulans by conquering much of their territory in the wake of the death of Shinzon and then the destruction of Romulus. The Klingons have also conquered the Gorn, the Orions, and the Nausicaans. Evidently, Klingons are seen joining Starfleet, if unlocked. The Federation-based mission "Past Imperfect" gives a possible answer to how the Augment virus' alterations was eventually cured by revealing that the mad Klingon Admiral B'Vat had kidnapped Miral Paris and brought her to the past, where they used her Klingon/Human DNA to manufacture a cure.